This invention relates to a charged particle beam apparatus and an aberration corrector used in a charged particle beam apparatus.
In recent years, an electron microscope aberration correction technology using a multipole aberration corrector is already becoming popular for use with a transmission electron microscope (TEM) and a scanning transmission electron microscope (STEM). In a high acceleration TEM and STEM having electron energy of 100 kiloelectron volt or higher, a spherical aberration is corrected through use of the above-mentioned technology, to thereby allow observation with an ultrahigh resolving power under 0.1 nanometer, which has been difficult so far. Also in a scanning electron microscope, achieving a higher resolving power is approaching a limit with related-art optimization of the shape of an objective lens and related-art optimization of energy on an electronic path, for example, retarding or boosting. Therefore, in the same manner as for the TEM and STEM, an aberration correction using a multipole aberration corrector is a highly expected technology as means for improving a resolving power particularly in a low energy region exhibiting energy of 1 kiloelectron volt or lower.
However, the aberration correction using a multipole aberration corrector differs from the high acceleration TEM and STEM in that a chromatic aberration is as or more liable to be a main factor that restricts the resolving power as or than the spherical aberration because an electron beam used by an SEM has low energy. Hence, a chromatic-and-spherical aberration corrector having a configuration different from that of a hexapole spherical aberration corrector widely used for the high acceleration TEM and STEM is demanded for the SEM and a low acceleration TEM and STEM (in particular, having energy of 30 kiloelectron volt or lower).
To that end, there have been proposed so far not only an antisymmetric fourfold quadrupole chromatic-and-spherical aberration corrector, which was proposed by H. Rose and the effectiveness of which was empirically confirmed by J. Zach et at, but also a chromatic (spherical) aberration corrector having a plurality of configurations (PTL 1). Those aberration correctors include a multistage quadrupole-octupole lens having two or more stages as a basic configuration. In particular, at least a two-stage multipole within a multistage multipole that forms the above-mentioned aberration corrector is characterized by being complex electromagnetic quadrupoles each configured to cause quadrupole fields of a magnetic field and an electrostatic field to occur in the same spot in order to carry out chromatic aberration correction. The complex electromagnetic quadrupoles including those multipoles may be further able to cause an octupole, hexapole, or dipole field to occur simultaneously in order to correct an aperture aberration up to the third order, for example, a spherical aberration. Therefore, respective multipoles of a quadrupole-octupole chromatic-and-spherical aberration corrector may often include not only the above-mentioned complex electromagnetic quadrupoles but also other multipoles formed of twelve poles in actuality.
As an example of the related art, a configuration of the antisymmetric fourfold quadrupole chromatic-and-spherical aberration corrector proposed by H. Rose is illustrated in FIG. 8 as a basic example, and its operation principle is described below. A chromatic-and-spherical aberration corrector 6 (hereinafter referred to simply as “aberration corrector 6”) described above includes a multipole 20, multipoles 21 and 22, multipoles 23 and 24, and a multipole 25 in four stages. As described above, complex electromagnetic quadrupoles configured to cause magnetic quadrupoles 21 and 23 and electrostatic quadrupoles 22 and 24 to occur in the same multipoles are formed in two inside stages of the four stages. The multipoles 20 and 25 before and after those complex electromagnetic quadrupoles mainly aim at guiding electrons to a trajectory suitable for chromatic aberration correction at complex electromagnetic quadrupoles 21-22 and 23-24, and an effective chromatic aberration correction itself is conducted through use of the complex electromagnetic quadrupoles 21-22 and 23-24 in two stages. Electrostatic quadrupoles are used as the multipoles 20 and 25 before and after the complex electromagnetic quadrupoles by J. Zach et al., but the multipoles 20 and 25 may be magnetic quadrupoles.
More specifically, electrons parallelly enter from the left of the aberration corrector as illustrated in FIG. 8, pass along trajectories 100x and 100y, and parallelly exit through a termination of the aberration corrector. An electron beam is also set to exhibit the same axis offset distance from a center axis at times of entering and exiting the aberration corrector. The aberration corrector is further designed to allow telescopic use without exerting an influence on an optical system outside the aberration corrector when the aberration corrector is turned on and off. In the inside of the aberration corrector 6, the electron beam entering from the left first branches off to an x-trajectory 100x and a y-trajectory 100y at the first-stage quadrupole 20, and the x-trajectory 100x passes through the second-stage complex electromagnetic quadrupole 21-22 away from the center axis. Meanwhile, the y-trajectory 100y is adjusted to pass through substantially the center of the complex electromagnetic quadrupole. When a rectangular field approximation is assumed as a field distribution, motion of electrons inside the second-stage complex electromagnetic quadrupole is described by equations of motion of Expression (1):
                                          x            ″                    =                                    -                                                θ                  2                                                  L                  2                  2                                                      ⁢            x                          ⁢                                  ⁢                              y            ″                    =                                    +                                                θ                  2                                                  L                  2                  2                                                      ⁢            y                                              Expression        ⁢                                  ⁢                  (          1          )                    
where L represents a pole length, and θ represents a parameter indicating a quadrupole field strength as expressed by Expression (2):
                    θ        =                                                            θ                M                2                            +                              θ                E                2                                              =                                    1              L                        ⁢                                                                                                                              2                        ⁢                        e                                                                    m                        ⁢                                                                                                  ⁢                                                  Φ                          0                                                                                                      ⁢                                      ψ                    2                                                  -                                                      ϕ                    2                                                        Φ                    0                                                                                                          Expression        ⁢                                  ⁢                  (          2          )                    
where θE and θM represent parameters indicating an electrostatic quadrupole strength and a magnetic quadrupole strength, respectively, as expanded in Expression (2).
In each of coefficients ψ2 and Φ2 within the third term of Expression (2), a distribution of a magnetic potential and an electrostatic potential of the quadrupole field is expressed by Expression (3).Ψ2(x,y)=2ψ2xy Φ2(x,y)=ϕ2(x2−y2)  Expression (3)
In Expression (2), e and m represent a charge and a mass of electrons, and Φ0 represents energy of the electrons passing through the aberration corrector. In consideration of a distribution of electron energy, Expression (2) is transformed as Expression (4) for electrons having the energy of Φ0+δΦ.
                                                                        θ                +                δθ                            =                            ⁢                                                1                  L                                ⁢                                                                                                                                                          2                            ⁢                            e                                                                                m                            ⁡                                                          (                                                                                                Φ                                  0                                                                +                                δΦ                                                            )                                                                                                                          ⁢                                              ψ                        2                                                              -                                                                  ϕ                        2                                                                    (                                                                              Φ                            0                                                    +                          δΦ                                                )                                                                                                                                                                    ≈                            ⁢                                                                    1                    L                                    ⁢                                                                                                                                                                        2                              ⁢                              e                                                                                      m                              ⁢                                                                                                                          ⁢                                                              Φ                                0                                                                                                              ⁢                                                      ψ                            2                                                                          -                                                                              ϕ                            2                                                                                Φ                            0                                                                                                                                              -                                                                                                      ⁢                                                δϕ                                      2                    ⁢                                          LΦ                      0                                                                      ⁢                                  (                                                                                    1                        2                                            ⁢                                                                                                    2                            ⁢                            e                                                                                m                            ⁢                                                                                                                  ⁢                                                          Φ                              0                                                                                                                          ⁢                                              ψ                        2                                                              -                                                                  ϕ                        2                                                                    Φ                        0                                                                              )                                                                                                                      ⁢                                                (                                                                                                                                          2                            ⁢                            e                                                                                m                            ⁡                                                          (                                                                                                Φ                                  0                                                                +                                δΦ                                                            )                                                                                                                          ⁢                                              ψ                        2                                                              -                                                                  ϕ                        2                                                                    (                                                                              Φ                            0                                                    +                          δΦ                                                )                                                                              )                                                  -                                      1                    2                                                                                                          Expression        ⁢                                  ⁢                  (          4          )                    
In conversion into the third term of Expression (4), δΦ is left up to its linear expression as a minute amount. Hence, its approximation is expressed as Expression (5):
                    δθ        =                              -                          δΦ                              4                ⁢                L                ⁢                                                                  ⁢                                  Φ                  0                                                              ⁢                                                    θ                M                            -                              2                ⁢                                  θ                  E                                                      θ                                              Expression        ⁢                                  ⁢                  (          5          )                    
where δθ represents a change of a quadrupole strength relative to a change (distribution) of electron energy. In other words, the two expressions of Expression (2) and Expression (5) mean that it is possible to adjust a dispersion (namely, Expression (5)) arbitrarily between positive and negative while maintaining a total strength (namely, Expression (2)) at a fixed value through use of the complex electromagnetic quadrupole. As a result, it is possible to form a convex lens having a negative dispersion in an x-direction with the second quadrupole 21-22 being an electromagnetic complex to impart a negative chromatic aberration to electrons on the x-trajectory 100x. In other words, dependence on the electron energy Φ0 is different between the electrostatic quadrupole and the magnetic quadrupole, and hence it can be said that, in the x-direction, a convex lens and a concave lens are formed with the magnetic quadrupole and the electrostatic quadrupole, respectively, and a convex lens having a negative dispersion is formed as a residual of cancellation of both to create a negative chromatic aberration. Meanwhile, as pointed out above, the y-trajectory 100y is adjusted to pass through the center of the second quadrupole 21-22, and therefore passes through the second quadrupole 21-22 almost without undergoing an influence of the quadrupole including an aberration.
In the same manner, the third quadrupole 23-24 being an electromagnetic complex is set so that the x-trajectory 100x passes through the center of the quadrupole and the y-trajectory 100y passes through a spot spaced apart from the center axis such that the third quadrupole 23-24 becomes antisymmetric with the second quadrupole 21-22. Those settings can impart a negative chromatic aberration to electrons on the y-trajectory 100y in contrast to the previous passage. The electrons that have branched off to follow an x-trajectory and a y-trajectory so far are stigmatically integrated at the fourth quadrupole 25 again to exit the aberration corrector 6. As a result, the above-mentioned antisymmetric fourfold quadrupole aberration corrector can provide a chromatic aberration that exhibits equal amounts in the x-direction and a y-direction and can therefore be adjusted rotationally symmetrically and arbitrarily between positive and negative. The antisymmetric fourfold quadrupole aberration corrector can further cancel out the above-mentioned chromatic aberration and a (positive) chromatic aberration of an objective lens 11 located optically downstream of the aberration corrector 6, to thereby achieve the chromatic aberration correction as an entire SEM optical system.
In addition to the above-mentioned correction, by superimposing and applying an appropriate octupole field to each of the multipoles in four stages of the aberration corrector 6, it is possible to correct a third-order aperture aberration including a spherical aberration. It is also possible to compensate first-order and second-order parasitic aberrations, which appear due to imperfection of the optical system from the inside and outside of the aberration corrector, by appropriately applying a hexapole field and a dipole field to the respective stages in the same manner. With the above-mentioned configuration, the chromatic aberration and the aperture aberration up to the third order can be corrected by the antisymmetric fourfold quadrupole aberration corrector.
In order to maintain a symmetric property with respect to the x-trajectory and the y-trajectory described above, the four-stage quadrupole of the aberration corrector further causes a fourth-order or higher-order aberration to be canceled out inside the aberration corrector by the symmetric property. Hence, the first stage 20 and the fourth stage 25, as well as the second stage 21-22 and the third stage 23-24, are arranged symmetrically with respect to a corrector center plane 15 inside the aberration corrector, and are simultaneously excited antisymmetrically. In other words, the above-mentioned four-stage multipole forms an antisymmetric fourfold quadrupole.